Recently, liquid crystal display devices, for example, have been widely used in liquid crystal television sets, monitors, mobile phones, etc., as flat panel displays that are thinner and lighter than conventional cathode-ray tubes. Such a liquid crystal display device includes an illuminating device (backlight) that emits light and a liquid crystal panel that displays a desired image by functioning as a shutter that blocks the light from a light source included in the illuminating device.
The above-described illuminating device generally includes optical elements including, for example, an optical sheet, such as a prism sheet, and an optical waveguide to improve the utilization efficiency of light from the light source, to irradiate the liquid crystal panel with planar illuminating light, or to increase the brightness of the illuminating light.
To evaluate the optical characteristics of such an optical element, it is very important to determine a refractive index of the optical element. This is because the higher the determination accuracy of the refractive index of the optical element, the easier an illuminating device having desired optical (luminous) characteristics can be produced in the case where the optical element is used in the above-described illuminating device.
As a refractive index measuring method according to the related art, a method of measuring a refractive index of a test subject (solid sample) by a total reflection method using a prism has been proposed in, for example, PTL 1 below. More specifically, in this refractive index measuring method according to the related art, the relationship between an incident angle of light incident on a reflection surface of a prism and the intensity of light reflected by the reflection surface is determined in each of a state in which a test subject is in contact with the reflection surface and a state in which only air is in contact with the reflection surface. In this refractive index measuring method according to the related art, the relationship between the incident angle and the reflected light intensity in the state in which the test subject is in contact with the reflection surface is normalized by using the relationship between the incident angle and the reflected light intensity in the state in which only air is in contact with the reflection surface. Then, the refractive index of the test subject is measured on the basis of the normalized relationship between the incident angle and the reflected light intensity.